1. Field of the Invention
This invention relates to a battery charging apparatus for a chargeable battery which is suitable for a video camera and the like.
2. Description of the Prior Art
For example, existing portable video cameras use a chargeable battery a nickel-cadmium type or a nickel-hydrogen type as an operating power source. The capacity of such a chargeable battery is about 600 mA/h and 1000 mA/h. A charging of the chargeable battery is performed by a dc power source which is obtained by rectifying an ac power source. In such charging, a constant current as shown in FIG. 1A is supplied to the chargeable battery. In addition, in order to make up for a decrease of charge due to self-discharge, even after a completion of charging, a charging current as shown in FIG. 1B is supplied to the chargeable battery.
According to the charging method mentioned above, even after a completion of charging, the same charging current as that in charging is supplied to the chargeable battery. In the meantime, since the amount of self-discharge is not great, it is not necessary to charge with the current as shown in FIG. 1B. Therefore, charging is performed by a current as shown in FIGS. 2A and 2B. That is, in a quick charging situation, a relatively large constant current as shown in FIG. 2A is supplied to the chargeable battery. After a completion of charging, in order to make up for the self-discharged charge, a relatively small constant current (refer to FIG. 2B) as compared with that in the quick charging is supplied to the chargeable battery.
FIG. 3 shows a circuit block diagram of a battery charging apparatus to supply the chargeable battery with such a current as shown in FIGS. 2A and 2B. In addition, charging to the chargeable battery is made either by quick charging for the short charging time or by a normal charging for the normal charging time. For example, if charging is normal, then a control signal supplied from a control block 101 activates a charging block 102 to flow a current I, for the normal charging to charge the chargeable battery 103.
In addition, whether the chargeable battery 103 is fully charged or not is detected in a full charging detecting block 104. If the full-charge detecting block 104 detects that the chargeable battery 103 has already been fully charged, a detection signal DS is supplied from the full-charge detecting block 104 to the control block 101. A control signal for controlling the amount of the current is supplied from the control block 101 to the charging block 102. The current to be supplied to the chargeable battery 103 becomes a relatively small constant current as shown in FIG. 2B.
If charging by quick charging, then a control signal output from the control block 101 activates a charging block 105 to flow a current I2 for a quick charging to charge the chargeable battery 103. In the same way as mentioned above, if the full-charge detecting block 104 detects that the chargeable battery 103 is fully charged, then a relatively small constant current is supplied to the chargeable battery 103.
Nevertheless, even if such a circuit configuration has been achieved, the chargeable battery is forced to be charged at all times.
As mentioned above, if the chargeable battery is charged at all times, there occurs an over-charge and the cell characteristics of the battery could be degraded. Moreover, if a small charging current is supplied at all times after the full-charge, then the drop in charge efficiency could result with no appropriate charging.
In addition, a battery charging apparatus having a plurality of charging modes such as a quick charging mode, a normal charging mode, or the like, requires circuits equal in number to the number of charging modes. Therefore, the number of parts will also increase to prevent the battery charging apparatus from being miniaturized. Moreover, if an integrated circuit (IC) is adopted in the control block, then pins of the IC are used for the quick charging and normal charging circuits. However, there is a limit to the number of the pins of the IC. For example, pins reserved for other signal lines could be used for the charging circuits to restrict the function of the IC itself.